In modern satellite systems, and in other applications, there is a need for radio frequency coaxial switches that perform at ever-higher frequencies. In the prior art, the design strategy for a coaxial switch has been to match the RF paths in as wide a frequency band as possible, such that a few designs could cover all of the frequency bands of commercial interest. However, prior art switch designs have demonstrated poor performance at high frequencies, mostly due to high mismatch losses.
The performance parameters of a coaxial RF switch are the RF power handling, the return loss and isolation; insertion loss is usually an outcome of design features imposed to achieve the desired RF power handling, return loss and isolation. Most coaxial switches, at high operating frequencies, are low RF power devices and therefore the RF power handling is not an important design driver. Isolation is a function of the RF channels, and is relatively easily predicted. However, return loss is more difficult to model, particularly in switches having a complex geometry, and is therefore more difficult to improve in a wide band RF switch design.
The majority of prior art coaxial RF switches that are impedance-matched to a wide frequency band of RF signals do not perform well at frequencies in excess of 30 GHz.
In addition, even relatively good performance individual switches, in terms of reflection, when cascaded, as required by the switching systems used on communication satellites for example, end-up as low performance assemblies. For example a cascaded assembly of N switches each having X return loss (in dB) will have an overall Y return loss (in dB) given by:Y=X+10·log10 N   (1)Equation (1) shows that the overall reflection of the assembly will deteriorate by 10·log10 N dB, which in the case of 6 cascaded switches for example, means almost 8 dB. This deterioration in performance for the assemblies containing cascaded switches is pushing the requirements on each individual switch higher by at least the same amount.
The low performance of the prior art coaxial switches at higher frequencies is due to the fact that as the frequency increases the wavelength decreases and discontinuities that were transparent for lower frequencies become important, in terms of the reflected signal. Therefore as the frequency increases in a wide frequency band, in order to reduce reflection, one needs high precision parts and very accurate positioning of the moving conductors inside the RF channels of the switch. These requirements become more stringent in complex switch structures, for example T-switch structures, which impose some transmission line discontinuities by their very nature.
The required precision of the switch parts and/or their accurate positioning inside the RF channels of the switch can be reduced where a specific switch will only be used in a limited frequency band around the commercially required frequency and hence will be required to be well matched only in this narrow frequency band. The development of coaxial switches with very good reflection performance around all the required frequencies is however prohibitive for switch manufacturers due to the high cost of producing switch parts requiring high dimensional diversity.